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ASE PS2 Test Preparation Guide: Brakes


10/10/2011
By Larry Carley

 

● DISC BRAKES — Use a pair of friction pads to squeeze against a flat disc or rotor when the brakes are applied. Most cars and light trucks since the early 1970s have front disc brakes, and many have rear disc brakes also. Disc brakes provide more stopping power than drum brakes, better cooling and fade resistance.

● BRAKE PADS — These are the friction linings used with disc brake systems. The friction materials used on the pads and shoes will vary depending on the application. The friction material may be nonasbestos organic (NAO), ceramic, low-metallic or semi-metallic. On some applications, the inner and outer pads may use different friction materials.

The friction material on a brake pad is usually molded to the steel backing plate, but may also use rivets for attachment. Some pads have internal noise dampening shims while others use noise suppression shims on the back of the pad. Slots and chamfers on the pads are also used to control noise.

Brake pads wear out and have to be replaced when their thickness reaches minimum specifications or a wear indicator makes noise. On most vehicles, the front pads wear two to three times faster than the rear pads or linings. But on some newer vehicles with electronic brake proportioning, the rear linings may wear faster than the front. Replacement also may be necessary if the linings have been contaminated with grease, oil or brake fluid, or if the linings are unusually noisy. As a general rule, the friction material in replacement pads should be the same (or better) than the original pads. Follow the replacement and upgrade recommendations of your friction supplier. Pads are always replaced in pairs (both fronts or both rears) to maintain even braking.

● CALIPERS — Are mounted over the rotors to squeeze the brake pads against the rotors when the brakes are applied. Most calipers have one one or two pistons, but some have up to four or more. Most calipers are cast iron (though some are aluminum) and have steel or molded phenolic (plastic) pistons. Most calipers are a “floating” design with slides or bushings that allow the caliper to move sideways and center itself over the rotor when the brakes are applied. There are also “fixed” calipers that do not slide.

On vehicles with four-wheel disc brakes, the rear calipers may have a parking brake mechanism to hold the pads against the rotor. On some applications, a “mini-drum” inside the rear rotors is used for a parking brake.

Common problems with calipers include fluid leaks due to worn piston seals, and sticking, which may be due to corrosion around he piston or corroded slides or bushings. Uneven pad wear (inner pad worn more than the outer pad) is a common symptom of caliper sticking. Leaky, sticking or damaged calipers should be rebuilt or replaced. Many brake experts also recommend rebuilding or replacing the calipers on high-mileage vehicles when the brakes are relined to prevent problems later on.

Replacement calipers should have the same type of piston (steel or phenolic) as the original if only one side is being replaced. Mounting hardware such as pins, bushings and slides should also be replaced when changing calipers, and should be lubricated with a high temperature moly brake grease. “Loaded” calipers come complete pads and hardware for easy installation.

● ROTORS — Disc brake rotors may be solid or vented (cooling fins between the faces). Some rotors are directional (left and right are different). The design of the rotor cooling vanes can also differ depending on the application. Most rotors are made of cast iron while others use a “composite” design with a stamped steel center hat and cast iron disc. Composite rotors are lighter than cast rotors, but also can be more troublesome because they flex more than solid rotors, making them more sensitive to runout and vibration.
Replacement rotors should be the same type as the original, though one-piece cast rotors are available for many applications that originally use composite rotors. Like pads, rotors should be replaced in pairs to maintain even braking side-to-side.

Rotors must be replaced when worn to minimum service or discard specifications. New rotors also are needed if the rotors are cracked, severely corroded or have hard spots. Resurfacing used rotors can restore flatness and parallelism, and is recommended (though not absolutely necessary) when new pads are installed. On vehicles with “captured” rotors located behind the hub, replacement can be difficult and may require replacing the wheel bearings also.

● DRUM BRAKES — A type of brake system that uses a cylindrical drum as the friction surface for a pair of brake shoes. The shoes push outward against the inside of the drum to apply the brakes. Many vehicles still have drum brakes for the rear wheels.

● DRUMS — Usually made of cast iron, the drum provides a friction surface for the shoes to rub against. As the drums wear, they may become grooved, out-of-round, develop a bell-mouth shape or crack. If worn beyond maximum inside diameter (ID) or discard specifications, are cracked or have hard spots, replacement is necessary. Like rotors, drums should be resurfaced when new brake shoes are installed.

● SHOES — The friction linings inside a brake drum. A pair of shoes are used to push out against the drum when the brakes are applied. The friction linings may be molded or riveted to the shoes. Replacement is required when the lining thickness is worn to minimum specifications. Like pads, the type of friction material on the replacement linings should be the same type (or better) than the original.

● DRUM HARDWARE — The return springs, holddown springs, self-adjusters and other cables, clips or springs used in the brake assembly. Return springs that pull the shoes back away from the drum when the brakes are released may become weak with age, allowing the brakes to drag. Self-adjusters can become corroded and stick, causing increased pedal travel as the shoes wear. The drum hardware in high-mileage vehicles should be replaced when new shoes are installed.

● WHEEL CYLINDERS — The hydraulic component inside a drumbrake that pushes the shoes out against the drum. The wheel cylinder has two opposing pistons that move outward when pressure is applied. The wheel cylinder is mounted on the brake backing plate, and has dust seals over the pistons to keep out dust and water. Each piston has a cup-shaped seal for the fluid inside. Common problems with wheel cylinders include fluid leaks and sticking. Wheel cylinders can be rebuilt or replaced. Leaking fluid can contaminate the brake shoes, requiring their replacement also.

● MASTER CYLINDER — Is connected to the brake pedal. This component applies hydraulic pressure to the calipers and wheel cylinders when the brakes are applied. A pair of pistons inside the master cylinder create hydraulic pressure when the brake pedal is depressed. This pushes fluid through the brake lines to each wheel to apply the brakes. Fluid is stored in a reservoir atop the master cylinder. The master cylinder has two separate hydraulic circuits. Each circuit operates two of the four brakes (both fronts, both rears or a diagonal pair). This is a safety requirement so if one side fails, at least two brakes will continue to operate.
Wear in the master cylinder may allow fluid to leak past the piston or shaft seals. A symptom of a bad master cylinder is a brake pedal that slowly sinks to the floor when braking at a stop light. Leaks or failure to hold pressure require rebuilding or replacing the master cylinder. Rebuilding aluminum master cylinders is not recommended because of the anodized coating inside the piston cylinder. On some older vehicles with antilock brakes (ABS), the master cylinder may be part of the ABS modulator, and is very expensive to replace.

● PRESSURE DIFFERENTIAL VALVE — A safety switch that turns on the brake warning light if there’s a loss of pressure or fluid in either hydraulic circuit.

● PROPORTIONING VALVE — Reduces hydraulic pressure to the rear brakes to maintain proper brake balance. Usually located on or near the master cylinder. Some vehicles have a “load sensing” proportioning valve attached to the rear suspension to vary brake pressure according to vehicle load. A growing number of late model vehicles do not use a proportioning valve, but use the ABS system to control rear brake pressure.

● BRAKE FLUID — A glycol-based hydraulic fluid used in brake systems to apply the brakes. Two basic types are used: DOT 3 and DOT 4. DOT 4 has a higher temperature rating than DOT 3. Use the type specified by the vehicle manufacturer. Brake fluid absorbs moisture and becomes contaminated over time. This promotes corrosion inside the brake system and lowers the fluid’s boiling temperature increasing the risk of pedal fade and brake failure during hard use. Many experts recommend replacing the brake fluid when the brakes are relined or other major brake repairs are performed. Some type of bleeding equipment (pressure or vacuum) may be needed to get the air out of the lines when brake lines are opened or the fluid is changed.

● BRAKE HOSES & LINES — Carry hydraulic pressure from the master cylinder to each brake. Rubber hoses should be replaced if cracked or leaking. Steel lines may have to be replaced if severely corroded, leaking or damaged (use approved brake lines only, never any other kind of tubing). Different types of end fittings are used, so be sure the end fittings on the replacement hoses or lines match the original.

● ANTILOCK BRAKE SYSTEM (ABS) — Monitors wheel speed when braking to reduce the risk of wheel lockup and skidding on wet or slick surfaces. Also works with the Electronic Stability Control (ESC) system and/or Traction Control System (TCS) to monitor and improve vehicle traction and handling. ABS components include wheel speed sensors, a hydraulic modulator (which includes the ABS solenoid valves for each brake circuit, and may also be combined with the master cylinder on older, “integral” ABS systems), the ABS control module, pump, high-pressure accumulator and pump relay. Replacement of components is only necessary when a failure has occurred.

 

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